Charged particle accelerator with integral transformer and shielding means

ABSTRACT

A charged particle accelerator wherein a receptacle surrounding an accelerating tube is made of an insulator, a grounded conductor layer is provided on the outside surface of the receptacle, at least one transformer is formed by opposing a primary coil on the outside and a secondary coil on the inside with the wall of the receptacle held therebetween, a conductor layer is provided on the inside surface of the receptacle at the transformer forming portion, a corona shield is provided at an end part of the inside conductor layer, and the induced voltage of the secondary coil is rectified and smoothed so as to apply the resultant voltage to a Wehnelt cylinder as a bias voltage. Thus, a continuously variable bias voltage can be obtained by regulating the applied voltage on the primary side of the transformer.

United States Patent Minamikawa et al.

CHARGED PARTICLE ACCELERATOR WITH INTEGRAL TRANSFORMER AND SHIELDING MEANS Inventors: Yoshihisa Minamikawa; Susumu Ozasa; Shoji Kamimura, all of Katsuta; Yasushi Saito, Mito, all of Japan Assignee: Hitachi, Ltd., Japan Filed: June 28, 1974 Appl. No.: 484,339

Foreign Application Priority Data July 2, 1973 Japan 48-73699 1111. C1. H01j 29/96 Field of Search 315/57, 94, 70, 105; 250/403, 311; 313/447, 452, 359, 360, 313

11/1966 Spruck 250/311 X 11/1966 Spruck 250/311 X BEST AVAILABLE 001 3 [111 3,895,254

1451 July 15, 1975 3,808,498 4/1974 Fujisawa 250/311 X FOREIGN PATENTS OR APPLICATIONS 1,312,988 11/1962 France 315/94 925,963, 1 "4/1955 Germany 973,616 4/1960 Germany 250/311 Primary ExaminerPalmer C. Demeo Attorney, Agent, or Firm-Craig & Antonelli 5 7 ABSTRACT A charged particle accelerator wherein a receptacle surrounding an accelerating tube is made of an insulator, a grounded conductor layer is provided on the outside surface of the receptacle, at least one transformer is formed by opposing a primary coil on the outside and a secondary coil on the inside with the wall of the receptacle held therebetween, a conductor layer is provided on the inside surface of the receptacle at the transformer forming portion, a corona shield is provided at an end part of the inside conductor layer, and the induced voltage of the secondary coil is rectified and smoothed so as to apply the resultant voltage to a Wehnelt cylinder as a bias voltage. Thus, a continuously variable bias voltage can be obtained by regulating the applied voltage on the primary side of the transformer.

8 Claims, 4 Drawing Figures g pmgm 15 i975 F/6./ PRIOR ART FILAM ENT POWER SOURCE CHARGED PARTICLE ACCELERATOR WITH INTEGRAL TRANSFORMER AND SHIELDING MEANS BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates to a charged particle accelerator, and more particularly to means for supplying electric power to the high voltage portion of an accelerating tube which is used as the electron source of an electron microscope.

2. Description of the Prior Art In a prior art accelerator, as regards a bias voltage to be applied between a filament and a Wehnelt cylinder which is provided around the filament and which serves to regulate the amount of electron rays from the filament, the value of a bias resistance disposed between the filament and the Wehnelt cylinder is mechanically adjusted by an external control rod. The bias voltage is bestowed by the self-bias system owing to the bias resistance. The bias voltage V has the relation of V R] where 1 denotes the amount of electron rays emitted from the filament and R the bias resistance value. The amount of electron rays I is also a function of the bias voltage V. It has been accordingly impossible to pro vide an arbitrary bias voltage or a continuously variable bias voltage.

The alignment of the filament, the change-over operation in the case of employing a plurality of filaments, etc., are involved in the high voltage portion in addition to the change-over operation of the bias resistance. All the adjustments have hitherto been made mechanically by means of control rods. The use of the control rod, however, increases the chance of a creepage of current along the insulation portion which is most unstable in the high voltage situation. Therefore, the reliability for the application of a high voltage becomes less, and the structure becomes complicated.

SUMMARY OF THE INVENTION An object of this invention is to obviate the disadvantages of the prior art, and to provide a charged particle accelerator which reduces the creepage insulation portion and which can provide a continuously variable bias voltage.

The fundamental construction of the charged particle accelerator according to this invention lies in provision of a receptacle surrounding an accelerating tube, which receptacle is made of an insulator. In addition a grounded conductor layer is provided on the outside surface of the receptacle, at least one transformer is formed by oppositely disposing the primary coil on the outside and the secondary coil on the inside with the receptacle wall held therebetween, and the induced voltage of the secondary coil is rectified and smoothed so as to apply the resultant voltage to a Wehnelt cylinder as a bias voltage.

A further construction of the charged particle accelerator lies in that a conductor layer is provided on the inside surface of the receptacle at the transformer forming portion, and that its end part is shielded against corona.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of a prior art multistage accelerator which is the electron source of an electron microscope;

FIG. 2 is a sectional view of a multistage accelerator for an electron microscope as shows an embodiment of this invention;

FIG. 3 is a diagram showing a circuit for the bias voltage and filament heating portion of the device in FIG. 2, and

FIG. 4 shows another embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to facilitate understanding of this invention, the structure of a charged particle accelerating tube of the prior art will be stated more in detail.

FIG. 1 shows the schematic construction of the prior art charged particle accelerator. In the figure numeral 1 designates an accelerating tube, 2 a divided resistance, 3 a receptacle, 4 the internal space of the receptacle having an insulating gas atmosphere, 5 a supply and exhaust port for the insulating gas, 6 a Wehnelt cylinder for adjusting the amount of emitted electrons, 7 a filament cathode forming an electron or ion source, 8 an electron beam, 9 an electron microscope body, 10 a bias resistance determining the amplitude of the current supplied to the cylinder 6, 11 a control rod for changing over the bias resistance, 11 a control rod for aligning the filament, 12 a high voltage cable, 13 a cable head, 14 a filament transformer, and 15 a filament power source. A high voltage is applied from a high voltage generator (not shown) through the high voltage cable 12 as well as the cable head 13 to the Wehnelt cylinder 6. The electron beam 8 emitted from the filament 7 passes through the accelerating tube 1 to the electron microscope body 9. The heating of the filament 7 is executed by the filament power source 15 applying heating current through the filament transformer 14 disposed within the cable head.

Hereunder the preferred embodiments of this invention will be described in detail. FIG. 2 shows an embodiment of this invention, and is a transverse sectional view of a charged particle accelerator. In the figure the same symbols as in FIG. 1 indicate the same constituent members. In the embodiment the accelerating tube I is stored in the receptacle 31 which is made of an insulator, for example, an epoxy resin. Since the receptacle 31 is made of an insulating material, a voltage is in duced in the surface thereof by the high voltage applied to the accelerating tube, which is dangerous. In order to avoid the danger resulting therefrom and also to make the electric field of the insulator portion uniform, the outside surface is coated with a conductive paint 16, which is grounded. Further, in order to protect the conductive paint 16, it is coated with a vinyl paint 23. The high voltage is applied to the accelerating tube through the high voltage cable 12 passing through the cable head 13. The filament is heated by current supplied from the power source 15 through the transformer 14. As regards a bias voltage for the control cylinder, a transformer is provided which is composed of the primary coil 17 disposed at the top part of the receptacle 31, and the secondary coil 18 insulated therefrom by the receptacle wall. Power is supplied from a power source 19 to this transformer, and power thus induced in the secondary coil is fed as the bias voltage by a circuit shown in FIG. 3. More specifically, the pwer induced in the secondary coil 18 is converted to a DC. voltage through a smoothing circuit 20. The

DC. voltage is applied between the filament 7 and the Wehnelt cylinder 6 in the form of the bias voltage, so as to adjust the amount of the electron rays 8. Since the bias voltage is applied through the transformer, arbitrary and continuous adjustments are possible from outside the receptacle by merely varying the bias voltage. By way of example, bias voltages of 0.1 V to 2,000 V are obtained. The spacing between the primary coil 17 and the secondary coil 18 becomes small by slenderizing the top part of the receptacle 3], so that the transmission efficiency of the transformer is enhanced. Since the secondary coil is at the high voltage potential as illustrated in FIG. 3, it is feared that the electric field will be disturbed by the projection of the outward shape, etc., with respect to the grounded conductive paint on the outer surface of the receptacle 31. For this reason, the inner wall of the receptacle 31 on the secondary coil side is coated with a conductive paint 21 in close adherence thereto, and the conductive paint 21 is maintained at the same potential as the high voltage. A corona shield 22 is provided at the end of the conductor 21, so as to prevent the electric discharge between the insulation surface along the inner wall of the receptacle and the grounded portion on the microscope body 9 side. In consequence, the insulator of the receptacle which intervenes between the primary coil and the secondary coil lies in a uniform electric zone owing to the conductive paints l6 and 21 which are respectively applied on the outer and inner surfaces of the receptacle. The thickness of the insulator differs in dependence on the applied voltage. To take one example, it is about mm when an epoxy resin having a withstand voltage of IO kV/mm is used at an applied voltage of 100 kV. Considering the electrode shape and the safety factor, the thickness is determined upon the calculation of the electric field.

Since the accelerating tube 1 and the receptacle 31 need be detached from time to time for repair and reassemblage, the secondary coil is mechanically mounted on the top part of the accelerating tube and is separated from the inner wall of the receptacle. The merit of such structure is that the secondary coil whose output need be connected to the Wehnelt cylinder 6 and the filament 7 located within the accelerating tube can be directly joined by soldering, etc. If the secondary coil is mounted on the receptacle, a separable contact must be provided between the accelerating tube and the secondary coil. Accordingly, the structure of the top part of the accelerating tube becomes complicated, and troublesomeness is inevitably involved in the assembling job.

In general, when a high frequency above 500 Hz is employed for the driving power source of the foregoing transformer, the transmission efficiency of the transformer is enhanced. As the frequency becomes higher, however, the effect of the electrostatic shield becomes greater and thereby the transmission efficiency of the transformer becomes lower. Therefore, when a silver paint of high conductivity (10 Q-cm) is used as the conducutive paint 16 and 21 applied in the vicinity of the transformer portion, the conductive paint becomes the electrostatic shield. In order to prevent such electrostatic shield, a semiconductive substance having a specific resistance of above 3OQ-cm is deposited on the inner and outer surfaces of the receptacle in the vicin- 6 ity of the transformer over a region at least 1.5 times Wider than the full lengthof the primary coil or the secondary coil.

FIG. 4 shows another embodiment of this invention. In the figure the same symbols as in FIG. 2 indicate the same constituent members. In this embodiment three insulated transformers are provided. An insulated transformer 24 feeds power to a driving motor 28, to perform the alignment of the filament 7 integral with the Wehnelt cylinder 6. An insulated transformer 25 heats the filament 7. The secondary coils of these two transformers are fixed at the top part of the accelerating tube 1, and are separated from a receptacle 32. An insulated transformer 26 bestows a bias voltage between the filament and the Wehnelt cylinder through a smoothing circuit 27.

Since the insulated transformer 26 is situated at right angles with the center axis of the accelerating tube, its secondary coil cannot be made integral with the accelerating tube. Accordingly, the secondary coil 18" is mounted on the receptacle 32 and supplies power to the smoothing circuit inside the acclerating tube by means of a contact 29 provided on the side wall of the top part of the accelerating tube. By thus disposing the insulated transformers on the receptacle 32, electric power can be fed to the high voltage portion. Further, when an insulator, for example, an epoxy resin, is used for the receptacle surrounding the accelerating tube, the complicated shape as stated above can be readily formed by the molding job.

As thus far explained, according to this invention, at least one transformer whose insulator is a part of the receptacle made of an insulator is employed, so that the insulated control rod which is prone to creepage discharge is not required and that the alignment of the filament, the change-over of a plurality of filaments contained therein, etc., can be made by a motor, a relay or the like. In addition, a continuously variable voltage can be supplied to the Wehnelt cylinder. The invention has such remarkable effects.

What is claimed is:

1. In a charged particle accelerator including electron ray generating means having a Wehnelt cylinder provided in the vicinity of a filament in an accelerating tube and means for applying a bias voltage to said Wehnelt cylinder to adjust the amount of electron rays emitted from the filament,

the improvement comprising a receptacle surrounding said acclerating tube and made of an insulator, an outer grounded conductor layer provided on an outside surface of said receptacle, at least one transformer formed by a primary coil arranged on the outside of said receptacle and connected to said bias voltage means and a secondary coil arranged on the inside of the receptacle opposite said primary coil with the receptacle wall therebetween, and means connecting said secondary winding to said Wehnelt cylinder for rectifying the induced voltage of said secondary coil and applying said rectified voltage to said Wehnelt cylinder as said bias voltage, whereby a continuously variable bias voltage can be applied to said Wehnelt cylinder by regulating the voltage applied to said primary windmg.

2. The charged particle accelerator according to claim 1, further including an additional conductor layer 5 provided on an inside surface of said receptacle at the transformer forming portion, and means for shielding an end part of said additional conductor layer against corona.

is made cylindrical, and said primary coil and said secondary coil are concentrically disposed with respect to said top part.

6. The charged particle accelerator according to claim 1, further including means for establishing a constant field zone enclosing the portion of said receptacle contacting said outer conductor layer.

7. The charged particle accelerator according to claim 6, wherein said means for establishing a constant field zone includes a second conductive layer disposed receptacle said receptable opposite said outer conductive layer.

8. The charged particle accelerator according to claim 7, further including means for shielding an end part of said second conductor layer against corona. 

1. In a charged particle accelerator including electron ray generating means having a Wehnelt cylinder provided in the vicinity of a filament in an accelerating tube and means for applying a bias voltage to said Wehnelt cylinder to adjust the amount of electron rays emitted from the filament, the improvement comprising a receptacle surrounding said acclerating tube and made of an insulator, an outer grounded conductor layer provided on an outside surface of said receptacle, at least one transformer formed by a primary coil arranged on the outside of said receptacle and connected to said bias voltage means and a secondary coil arranged on the inside of the receptacle opposite said primary coil with the receptacle wall therebetween, and means connecting said secondary winding to said Wehnelt cylinder for rectifying the induced voltage of said secondary coil and applying said rectified voltage to said Wehnelt cylinder as said bias voltage, whereby a continuously variable bias voltage can be applied to said Wehnelt cylinder by regulating the voltage applied to said primary winding.
 2. The charged particle accelerator according to claim 1, further including an additional conductor layer provided on an inside surface of said receptacle at the transformer forming portion, and means for shielding an end part of said additional conductor layer against corona.
 3. The charged particle accelerator according to claim 1 wherein a protective vinyl film is provided on said grounded outer conductor layer.
 4. The charged particle accelerator according to claim 1 wherein said secondary coil is mechanically mounted at a top part of said accelerating tube and is spaced from the insulator receptacle.
 5. The charged particle accelerator according to claim 1 wherein a top part of the insulator receptacle is made cylindrical, and said primary coil and said secondary coil are concentrically disposed with respect to said top part.
 6. The charged particle accelerator according to claim 1, further including means for establishing a constant field zone enclosing the portion of said receptacle contacting said outer conductor layer.
 7. The charged particle accelerator according to claim 6, wherein said means for establishing a constant field zoNe includes a second conductive layer disposed receptacle said receptable opposite said outer conductive layer.
 8. The charged particle accelerator according to claim 7, further including means for shielding an end part of said second conductor layer against corona. 